<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <meta http-equiv="X-UA-Compatible" content="ie=edge">
    <title>Document</title>
</head>
<body>
    <p>你还可以输入<em id="em" style="color: red">0</em>/300</p>
    <textarea  cols="30" rows="10" id="txt"></textarea>
    <script>
        //onkeydown-->键盘按下去触发
        var em =document.getElementById("em");
        var txt =document.getElementById("txt");
        txt.onkeydown =function(){
            var len =this.value.length;
            em.innerHTML =len;
        } 
    </script>
</body>
</html>